1. Field of the Invention
This invention relates to the testing of inductively loaded transmission lines to identify incorrect loading on such lines.
2. Description of the Prior Art
Insertion of load coils at regular intervals greatly improves the voice-transmitting properties of long twisted-pair cables. The most common loading arrangement in telephone systems places 88 millihenry load coils every 6,000 feet on loops longer than 18,000 feet. Using this arrangement as a basis, there are over five million loaded telephone loops and at least several million loaded trunk facilities in the continental United States.
Faulty loading caused by missing or excessive load coils, incorrect spacing of coils, or damaged coils can cause voice transmission to be poorer than if the loop were correctly loaded and in some cases poorer than if the loop were not loaded. The prior art discloses test equipment for identifying such faulty loading. A typical piece of prior art test equipment includes a cathode ray tube for displaying a pattern representing, as a function of frequency, the impedance characteristic of a line under test. At the same time, the tube displays a second pattern which represents, as a function of frequency, the impedance characteristic of a lattice network. This network is made up by the test equipment operator from a kit to cause the second pattern to approximate that of the line under test. When a good match between the two patterns is achieved, then the configuration and values of the network elements are indicative of the composition of the line under test.
Although the above-described equipment has been used successfully, it has been found that producing a reasonable match of patterns is time consuming and somewhat of an art. Equipment which produces rapid results with relatively inexperienced operators would permit better overall telephone service.
Apparatus and methods which permit incorrect line loading to be identified by relatively inexperienced operators are disclosed in U.S. patent application Ser. No. 787,852, filed on even date herewith by F. D. Blum, D. L. Hester and J. T. Peoples. In accordance with the disclosure in that application, immittance magnitudes as a function of frequency of the input immittance of a line under test are produced. The frequencies at which local maximum and minimum values occur in the magnitudes are detected and a ladder network comprising series connected inductors and shunt connected capacitors is then synthesized wherein the pole and zero frequencies of the network approximate in value those previously detected. The values of the inductors and capacitors of the synthesized network represent the values of the loading inductors and the line spacing between the inductors, respectively.